Several arrangements exist for accessing a plurality of telephone central office lines from a single station set. The oldest and most commonly used arrangement is an electromechanical key telephone where several central office lines are brought into a single station, connection between the central office lines and the telephone apparatus being made mechanically through a switch. In a typical key telephone, several central office lines are brought in parallel into a single telephone station, and the station is equipped with keys and switches in order to connect each of the central office lines, one line at a time, with a single telephone apparatus. Because several stations typically must have access to the same central office lines a key telephone system includes a distribution box which distributes the tip and ring wires from each central office line, in parallel, to the several telephone stations. When several stations share several central office lines, visual indicators are associated with the keys for signaling purposes to identify, for example, which central office line is ringing, which line is engaged, or which line is on hold.
An alternative arrangement is the electronic key system in which each telephone apparatus is accessed by only one pair of tip and ring wires. Switching between the various central office lines and the telephone stations is done in a central, electronically controlled switching matrix located in an electronic key service unit. Typically, the telephone station contains keys which enable the user to identify to the switching matrix which central office line the user wishes to have connected to that station. Signaling and visual status indicators are made available to stations via additional wires, which carry the necessary electronic signals to perform the central office line selection and control, and to turn on and off the visual indicators.
Although key-system services are popular among a large class of customers, the provision of specialized wiring and distribution arrangements for key-system stations is expensive and inflexible. A recent approach to the provision of key-system services is to connect each station via an individual line to a stored program controlled, central office or private branch exchange (PBX) switching system and to have the central control of that system coordinate the processing of calls to and from members of key-system groups. Accordingly, no specialized wiring or distribution arrangements are required for the key-system stations. Key-system group control becomes much more complex in such switch-based arrangements, however, when the control is distributed to a plurality of switching modules to reduce the involvement of the system central control in per-call tasks. This is particularly true in typical applications where it is important that the members of a key-system group can be spread across a number of switching modules to enhance reliability and for administrative convenience in changing group members.
In view of the foregoing, a recognized problem in the art is the difficulty in controlling key-system groups from a switching system without relying on a central control.